Ligand Protonation Triggers H2 Release from a Dinickel Dihydride Complex to Give a Doubly “T”-Shaped Dinickel(I) Metallodiradical

Abstract

The dinickel(II) dihydride complex (1K) of a pyrazolate-based compartmental ligand with β-diketiminato (nacnac) chelate arms (L−), providing two pincer-type {N3} binding pockets, has been reported to readily eliminate H2 and to serve as a masked dinickel(I) species. Discrete dinickel(I) complexes (2Na, 2K) of L− are now synthesized via a direct reduction route. They feature two adjacent T-shaped metalloradicals that are antiferromagnetically coupled, giving an S=0 ground state. The two singly occupied local (Formula presented.) type magnetic orbitals are oriented into the bimetallic cleft, enabling metal–metal cooperative 2 e− substrate reductions as shown by the rapid reaction with H2 or O2. X-ray crystallography reveals distinctly different positions of the K+ in 1K and 2K, suggesting a stabilizing interaction of K+ with the dihydride unit in 1K. H2 release from 1K is triggered by peripheral γ-C protonation at the nacnac subunits, which DFT calculations show lowers the barrier for reductive H2 elimination from the bimetallic cleft.